This application claims the priority of Korean Patent Application No. 10-2004-0069149, filed on Aug. 31, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a method of fabricating a laser diode, and more particularly, to a method of fabricating a laser diode using self-alignment.
2. Description of the Related Art
Laser beams of a semiconductor laser diode are used in varied fields, such as optical communications, multiple communications and space communications. A semiconductor laser is widely used as a light source for data transmission and data recording or reading in many apparatuses, such as compact disk players (CDPs) and digital versatile disk players (DVDPs).
The broad application of a semiconductor laser diodes are because such diodes can maintain an oscillation characteristic in a limited space, be scaled down and, above all, provide a small threshold current for laser oscillation. As semiconductor lasers are applied in increasingly varied fields, the requisition for semiconductor laser diodes having a lower threshold current increases. That is, a semiconductor laser diode having excellent characteristics, which allows low-current oscillation and long life span, is required.
FIG. 1 is a cross-sectional view of a conventional semiconductor laser diode. The semiconductor laser diode includes a ridge wave guide structure that provides a reduced threshold current for laser oscillation.
Referring to FIG. 1, an n-GaN lower contact layer 12, which is divided into a first region R1 and a second region R2, is stacked on a sapphire substrate 10. In the first region R1, an n-GaN/AlGaN lower clad layer 24, an n-GaN lower waveguide layer 26, an InGaN active layer 28, a p-GaN upper waveguide layer 30, and a p-GaN/AlGaN upper clad layer 32 are sequentially stacked on the n-GaN lower contact layer 12. In this case, the refractive index of the n-GaN/AlGaN lower clad layer 24 and p-GaN/AlGaN upper clad layer 32 is lower than that of the n-GaN lower waveguide layer 26 and p-GaN upper waveguide layer 30. Also, the refractive index of the n-GaN lower waveguide layer 26 and p-GaN upper waveguide layer 30 is lower than that of the InGaN active layer 28. In an upper central portion of the p-GaN/AlGaN upper clad layer 32, a protruding ridge 32a having a predetermined width is formed, providing a ridge waveguide structure. A p-GaN upper contact layer 34 is formed on the top surface of the ridge 32a. On the p-GaN/AlGaN upper clad layer 32, a buried layer 36 having a contact hole is formed as a passivation layer. The contact hole 36a of the buried layer 36 corresponds to a top portion of the upper contact layer 34, and an outer portion of the contact hole 36a overlaps an outer portion of the top surface of the upper contact layer 34.
A p-type upper electrode 38 is formed on the buried layer 36 and contacts the upper contact layer 34 through the contact hole 36a of the buried layer 36. On the n-GaN lower contact layer 12, an n-type lower electrode 37 is formed in the second region R2 that is lower than the first region R1.
The ridge waveguide structure provided on the upper clad layer 32 limits a current supplied to the active layer 28, thereby reducing a width of a resonance region for laser oscillation formed in the active layer 28. Thus, the ridge waveguide structure stabilizes a transverse mode characteristic and lowers an operating current.
In a process for providing the foregoing ridge waveguide structure, a contact hole corresponding to the top surface of a ridge can be formed in a buried layer covering an adjacent region of an upper clad layer by performing photolithography using a mask. However, this photolithography results in low precision of a fabrication process and an insufficient contact area between an upper contact layer and a p-type upper electrode. Hence, the operating voltage of the laser device is elevated, and any path through which heat generated during driving is emitted cannot be secured.
For this reason, self-alignment is preferred as a method of forming a contact hole in a laser diode. WO No. 2000/52796 teaches a method of forming a self-aligned contact hole by liftoff using selective solution of materials. However, in this method, a buried layer having an excessively large thickness cannot be lifted off. Thus, the thickness of the buried layer to be lifted off should be limited below a predetermined value. In particular, because the liftoff method utilizes a difference in solubility between materials, only a restricted range of materials can be used to form the buried layer.
In another conventional method using self-alignment, an etchback process is performed such that contact hole is formed in a buried layer corresponding to the top surface of a ridge. Specifically, a planarized photoresist is formed on the entire wafer in which the buried layer is formed on the ridge. Thereafter, a portion of the photoresist, which is formed on the ridge, is etched back using dry etching so that the contact hole corresponding to the top surface of the ridge can be formed. In this technique, since there is no etch stop layer between the buried layer and the ridge, it is difficult to grasp an etch stop point during the etchback process. Also, a portion of the buried layer, which is disposed on the ridge portion and exposed by the dry etchback process, should be removed using wet etching, not dry etching, because the dry etching may damage an upper contact layer disposed as an upper portion of the ridge. However, in this case, an etchant used for the wet etching penetrates between the photoresist and the buried layer, thereby over-etching the buried layer toward the lateral surfaces of the ridge.